A methodology for characterizing the hydraulic effectiveness of an annular low-permeability barrier

Excavation works in urban areas are often performed in compartments enclosed by means of grouting or structural walls that provide hydraulic isolation and mechanical stability. These functions are sensitive to the presence of discontinuities (apertures in the enclosing wall), which may cause flooding of the excavation and dragging of soil particles. The objective of this work is to develop a methodology to evaluate the hydraulic efficiency of the man-made low-permeability barrier and to identify possible discontinuities, so as to repair them before excavating. To this end, we consider the drainage of a circular excavation enclosed by an annular low-permeability barrier that may present an aperture. We solve the problem numerically in dimensionless form. The overall hydraulic effectiveness of the enclosing system can be easily characterized by monitoring pumping rate and drawdown. However, several piezometers are needed to identify the location of an aperture. The proposed method for characterizing the effectiveness of a low-permeability barrier is applied to several real construction sites. Although little information was available in one case, the method presented here gives results that agree well with those obtained from a numerical model. In another case where data from several piezometers was available, the region of an existing aperture in the low-permeability barrier was localized.